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Thousands of species of microbes make the giant leap across the
Pacific Ocean from Asia to North America by hitching rides on
dust plumes high up in the atmosphere, a new study finds.

The findings, detailed in the December issue of the journal
Applied and Environmental Microbiology, have implications for
understanding the transport of both air pollutants and organisms,
and could influence the way we view the atmosphere's relationship
to Earth's life.

"I think we're getting close to calling the atmosphere an ecosystem,"
said lead author of the paper describing the research David J.
Smith, who recently earned his PhD at the University of
Washington. "Until recently, most people would refer to it as a
conveyor belt, or a transient place where life moves through. But
the discovery of so many cells potentially able to adapt to
traveling long distances at high altitudes challenges the old
classification."

Small life, small world

Smith and his colleagues sampled two large dust plumes that
wafted over the ocean from Asia in spring of 2011 at Mount
Bachelor in the Cascade Mountains of central Oregon, according to
a university release. It's estimated that each year such dust
plumes carry about 7.1 million tons of aerosols, tiny particles
like dust, other pollutants and even microbes that are suspended
in the air.

The team's initial search for microbes in the plumes relied on
traditional methods of culturing these life forms, and found only
18 species. That work was published in July.But the scientists
were able to gather enough DNA from the biological samples they
took to apply more fine-tuned methods to analyze samples. These
methods revealed a whopping 2,100 unique species in the plumes.

Of the microbes the team identified, about half were bacteria and
half
were fungi. Most of them originated from soils and were dead
once they reached North America, according to the release. Most
of the microbes also posed no threat to humans.

Many of the species traced by the study are found at low,
background levels on the West Coast, but their numbers spiked
when the plumes arrived. Because of this, the scientists likened
the microbes to other forms of air pollution that are also
present in the background, but that can become elevated during
particular events.

"I was very surprised at the concentrations. One might expect the
concentrations of cells to decrease with altitude based on
fallout and dilution," Smith said. "But during these plume
events, the atmosphere was pooling these cells just as it does
with other kinds of air pollution."

Biological final frontier

Two of the three taxonomic families of bacteria that the
scientists identified are known to adapt well to
harsh conditions, such as those in the
chilly upper troposphere, about 11 miles (18 kilometers)
above the Earth's surface. These bacteria can form spores that
allow them to essentially hibernate.

The scientists haven't seen much evidence for the growth of
microbes when aloft, but the microbes can
influence precipitation by acting as nuclei that water vapor
can glom onto to form rain and snow. Other studies have estimated
that microbes serve as the nuclei for about 30 percent of the
world's precipitation, the release noted.

The scientists hope to sample the upper atmosphere for more such
microbes, seeking a better understanding of how they interact
with that harsh environment and how they might affect the global
flow of life. But such efforts face challenges, the researchers
note.

"Because it is so difficult to get samples, I argue it's probably
the last biological environment on the planet to be explored,"
Smith said.